When users wish to communicate across a communications network such as an optical network, it is necessary for a path to be provisioned through the network between the two desired end points at the user locations. Thus a typical first step in setting up a connection between the users would be to issue a path set up request to the control layer of the network. Typically such a request would include information about the start and end locations of the path, and the kind of path required for example, and the bandwidth requirement. The request may also specify the type of protection which is required i.e. the method which is used to provide an alternative path should the primary path fail.
The control layer of the network then proceeds to determine whether a path between the start and end points is available and if so what the optimum path is. Clearly the routing algorithm for the path should attempt to determine an optimum route taking into account factors such as routing policies set by the network management layer, available links, or any existing traffic on links and available capacity on those links and the resilience of those links particularly with reference to the desired protection (for example are the links following a shared path). These constraints are typical for all networks.
For analogue networks such as optical networks however, additional constraints must be taken into account. For example, ageing of components and fibres must be taken into account and margin built into the system to allow for these effects during the lifetime of the path, when provisioning the path.
Thus, for example with reference to FIG. 1, in an optical network, a margin of several dB, (maybe of the order of 12 dB), must be left to allow for degradation of the signal in the analogue domain. Typical components of the margin break down into random effects such as polarisation mode dispersion (PMD) and polarisation mode loss (PML), configuration tolerances such as optical signal to noise ratio (OSNR) equalisation mismatch, and long term OSNR degradation, component characteristics such as ageing of components and potential fibre losses such as normal signal degradation with length of transit in fibre and an allowance for losses through in service splicing of the fibre. These margins must all be added together to provide a total system margin which must be built into the path during routing.
Particularly initially, these margins are, to some extent and for some wavelengths, unused. Thus, additional equipment, especially regenerators, has been inserted into the network and gives no immediate benefit. Indeed, calls may be routed less-directly than they need to be in order to go via a regenerator site.
Thus in a typical network, the network must effectively be over-provisioned with unused capacity in order (amongst other things) to ensure that these margins and appropriate protection paths are in place. Furthermore, the network has little flexibility in how to provision the path. For, example in addition to over provisioning, the path setup request typically will force the network to use a particular type of protection which may not be appropriate for the current network state. It will be understood that any steps that may be taken to improve network utilisation (i.e. to reduce the requirement for over-provisioning) will provide cost benefits since the financial return produced from a particular investment in network equipment can be improved.